docs: fix pip install command typo

[thehanslevi]

Summary:

• Fix one README installation command typo from pip isntall to pip install.

Validation:

• python3 -m pip isntall -e polyffusion/mir_eval fails with unknown command "isntall".
• /tmp/codex-proof-pr-20260619-polyffusion-venv/bin/python -m pip install --no-deps -e polyffusion/mir_eval succeeds in an isolated temporary venv.
• rg -n "pip isntall|pip install -e polyffusion/mir_eval|Installation" README.md -S
• git diff --check

Residual uncertainty:

• I did not run the project training/inference workflows; this only changes a README install command.